JSFiddle - React, Tailwind, and code Playground

JavaScript

data = [
    [1, 622, 782, 783, "2015-04-21"],
    [2, 622, 782, 783, "2015-04-21"],
    [3, 622, 782, 783, "2015-04-22"],
    [4, 622, 782, 783, "2015-04-23"],
    [5, 622, 782, 783, "2015-04-24"],
    [6, 622, 782, 783, "2015-04-28"],
    [7, 622, 782, 783, "2015-04-28"],
    [8, 622, 782, 783, "2015-04-29"],
    [9, 622, 782, 783, "2015-05-04"],
    [10, 622, 782, 783, "2015-05-05"]
];

dates = [];
for (i = 0; i < data.length; i++) { 
    dates.push(new Date(data[i][4]));
}
console.log(dates);

Array.prototype.max = function() {
    return Math.max.apply(null, this);
};

Array.prototype.min = function() {
    return Math.min.apply(null, this);
};


alert("Max: "+new Date(dates.max())+"\n\n"+
      "Min: "+ new Date(dates.min()));